On the surface of Lake Washington, construction workers on the new Highway 520 floating bridge forge ahead, even as divers perform underwater inspections to find out if the first batch of pontoons is still leaking.

Concrete was poured last week to add five columns to the giant end pontoon. These columns will support the future road deck as it slopes down from the Medina shore toward the lake surface.

“We’re confident enough that it’s fixable, we’re starting to build the columns on top of them,” said Jeff Carpenter, chief construction engineer for the state Department of Transportation (DOT).

Extensive cracking in the first batch appeared midyear during the latter stages of construction in a Grays Harbor casting basin. Hundreds of small and large cracks were sealed before the pontoons were towed to Lake Washington in early August.

But as of Tuesday, not all have been repaired, said DOT spokesman Steve Pierce. In some places water is still seeping in.

Senior state engineers say they’re comfortable with the integrity of the huge concrete structures because of the thinness of the cracks, assurances from outside experts, and experience repairing cracks on other bridges, including the 2009 Hood Canal Bridge.

Right now, the big 520 end piece, the first two main lengthwise pontoons, and six smaller buoyancy pontoons are being assembled on the lake. A total of 77 pontoons built in Grays Harbor and Tacoma will be fastened together with concrete grout, so they rise and fall slightly like one big barge on the lake.

Kiewit-General is building the pontoons under a $367 million contract, and the floating part of the bridge is to open in late 2014, as part of the overall $4.13 billion crossing from Interstate 5 in Seattle to Interstate 405 in Bellevue.

Watertight pontoons are crucial to the success of the world’s longest floating bridge, which is supposed to last 75 years.

Results of this week’s underwater inspections will be forwarded to Gerwick, a marine consulting firm that will suggest further repair options.

Markings from earlier repairs cover the walls and decks of pontoons, as seen during a walk of the site last week.

A crystalline waterproof paste, which looks like stucco, was spread over hundreds of superficial cracks. Some cracks are normal when concrete dries, but an expert report found more cracks than expected.

More important are “structural” cracks wider than six-thousandths of an inch — the thickness of a sheet of paper. Epoxy filler was injected to fill structural cracks.

The worst cracks happened during post-tensioning, when steel bands were strung through the pontoons and tightened to compress the concrete. Portions of some end walls “pooched” outward, in places that didn’t have steel applying inward pressure, said Tom Skoro, a senior vice president of Kiewit-General.

Some cracks radiated to the decks and the bottoms.

Kiewit-General says its workers used epoxy to fill the bottom cracks from the interior when the pontoons were being built at Grays Harbor. But workers couldn’t reach these cracks from outside, because the pontoons sat on the basin floor. So Kiewit needs underwater inspections now to verify that they’re watertight.

The big end-piece W, where columns are being built, was still seeping after it reached Lake Washington, according to an Aug. 27 Kiewit letter to DOT.

Since then, all known cracks in the outer walls have been treated, said DOT project manager Julie Meredith. “The fact (water) it’s not coming through those cracks now is a very good sign,” she said Friday, while walking on a lengthwise pontoon.

Inside a lengthwise pontoon, a cluster of six small cracks allowed drops of ballast water to leak into one of its 52 interior cells. Engineers say that’s not a problem because the cells will soon be ballasted with gravel, not water. Two other cells nearby showed no apparent leaks.

Meanwhile, the state and Kiewit-General are negotiating who will pay for repairs and delay, initially estimated by one Kiewit memo to be 59 days. The company increased night shifts this summer to regain time, said Skoro. Transportation Secretary Paula Hammond has said the state is responsible for one error — when an interior corner broke apart under the strain of post-tensioning May 11, forcing retrofits to four large pontoons.

Will these kinds of cracks repeat themselves?

The answer will come when the next six pontoons, now being cast in Grays Harbor, are floated out next spring. They are now one-third complete, with no major cracks visible on exterior walls, and two sizable cracks in the less-critical cell walls, DOT says.

The 2009 Hood Canal Bridge shows that cracks early in the project can be overcome, officials said.

In that project, wall cracks appeared in the initial set of pontoons made by Kiewit-General in Tacoma, because of problems with curing temperatures. A change in casting methods reduced crack size in later rounds, DOT reported in an industry newsletter.

Kiewit or General have built pontoons for five floating bridges, erected both the high-rise and swing West Seattle bridges, and partnered on the 2007 Tacoma Narrows suspension bridge.

“The work we do in Washington is important to us,” said spokesman Tom Janssen, “and we expect to be here in the long haul.”